Size reduction of image pickup devices and size reduction of lens systems is greatly demanded in today's video camera market. Moreover, it is an important factor that electronic still-video cameras which have appeared in recent years with the spread of multimedia personal computers are inexpensive and small in size. On the contrary, for the manufacturers, it is necessary to reduce the cost in order to have competitive power in the market. In order to reduce lens systems in cost, it is necessary to provide a construction of lens system which has a minimum number of lens elements and is capable of maintaining performance the same as or higher than that of conventional lens systems while having a minimum number of lens elements.
For these reasons, it has conventionally been a problem how a zoom lens system is provided to be is small in size and high in resolution but has a fewer number of lens elements.
A conventional zoom lens system will be described.
FIG. 32 is a view showing the structure of a conventional zoom lens system for use in a video camera described, for example, in Japanese Laid-open Patent Publication Number Hei 6-109975. The zoom lens system shown in the figure comprises a first lens unit 321 serving as a condenser unit, a second lens unit 322 serving as a zooming unit, a third lens unit 323 serving as a condenser unit, a fourth lens unit 324 serving as a focusing unit, an equivalent glass plate 325 corresponding to a crystal filter or a faceplate of an image pickup element, etc., and an image forming plane 326.
The first lens unit 321 fixed in relation to the image forming plane 326 has imaging function, and the second lens unit 322 moved back and forth along the optical axis varies the magnification thereby to vary the focal length of the overall lens system. The third lens unit 323 which is a stationary unit condenses light diverged by the second lens unit 322, and the fourth lens unit 324 moved back and forth along the optical axis has focusing function. Moreover, the variation in position of the image plane caused by the movement of the second lens unit 322 during zooming is corrected by the movement of the fourth lens unit 324 so as to image in a fixed position, whereby the image plane is always maintained in a fixed position.
However, in the conventional zoom lens system as described above, what is called f-number is 1.6 or higher although the number of lens elements is ten and the zoom ratio is approximately 10.times.. Therefore, it cannot cope with the insufficient sensitivity which is a problem in reducing of image size. Moreover, although having a small number of lens elements, its overall length is comparatively large and it lacks compactness. For this reason, it is a problem that the conventional zoom lens system cannot fulfill the harsh demand for zoom lens systems for video cameras which is to be smaller in size and has higher performance. Moreover, it is difficult for the conventional zoom lens design technique to produce a zoom lens system which fulfills all of a large aperture, a high magnification, a smaller size and a higher resolution.